1. Technical Field
The present invention relates to a preform usable in molding a plastic vessel having an internal partition wall which is capable of resisting an internal pressure from a filled matter, and a process of making such a plastic vessel.
2. Background Techniques
When a preform is biaxially oriented, it is known in the art to provide a plastic vessel which has an open top portion as part of the preform, a cylindrical barrel portion formed below the top portion and having an internal diameter larger than that of the top portion, a shoulder portion connecting between the top and barrel portions, and a bottom portion formed to close the barrel portion at the end remote from the top portion. Many plastic vessels molded by the prior art generally have increased degrees of stretching in longitudinal and transverse axes and circular cross-sections in their barrel portions. In such plastic vessels, the internal stress from the internal pressure can be uniformly dispersed through the barrel portion in its circumferential direction since the barrel portion is of circular cross-section. Further, any distortion of the barrel portion due to the internal pressure from such as carbon dioxide gas can be fully prevented by the mechanical strength of the barrel portion which has been increased by crystallization from the biaxial orientation.
In case when a plastic vessel has its barrel portion of triangular or square cross-section or of oval cross-section, however, the internal pressure will concentrate on only a part of the side barrel wall, resulting in distortion of the side barrel wall which is weaker in mechanical strength.
In a further case of so-called wide-mouthed type plastic vessels in which the difference of internal diameter between the top and barrel portions in a plastic vessel is relatively small, such plastic vessels are inferior in mechanical strength even if they are biaxially oriented for crystallization, because the ratio of transverse stretch is small. Even if a widemouthed type plastic vessel is of circular cross-section, it is probably impossible to prevent certainly the distortion in the barrel portion of such a plastic vessel due to the internal pressure from the contents therein.
A proposal for increasing a wide-mouthed type plastic vessel in its mechanical strength is made in WO90/05674 which discloses a bottle comprising an internal spider which extends across the interior of the bottle and is biaxially oriented with the side wall of the bottle.
In the biaxial-stretch blow molding process for such a bottle, the smoothness of its biaxially stretching operation for a preform is obstructed by the presence of the internal spider, unlike the prior art biaxial orientation for cylindrical preforms. Since the internal spider is connected at one end to the inner side wall of the preform, that portion of inner side wall which is connected to the internal spider substantially resists the biaxial stretching step. It has been found that when the preform has been molded into a bottle, the outer side wall of the bottle tends to have a recess formed therein at that portion of the bottle which is connected to the internal spider. Such a recess will very damage the external appearance of the produced bottle.
In course of the inventors' study, it has been found that it is very difficult to form such an internal spider into a substantially uniform wall thickness when a bottle is molded. Unlike the conventional process for biaxially stretching only the side wall of the preform, the portions of the spider extending in various directions resist to each other on stretching such that the preform will be prevented from being stretched into a uniform wall thickness. In addition, it has been found that when the preform is stretched by a stretching rod along the longitudinal axis and by a blown air in the transverse direction within each of the partitions divided by the internal spider, the internal spider will be bias stretched in a particular orientation.
It was thus difficult that an actually usable bottle having its internal spider could be molded without various improvements.
It is therefore an object of the present invention to provide a preform which is optimum in molding a plastic vessel having an internal partition wall.
Another object of the present invention is to provide a process of molding a plastic vessel having an internal partition wall without damage to the external appearance thereof.